Fluorescence endoscopy involves introducing excitation light into the body and collecting the emitted fluorescence light by means of a probe that is inserted into the body to the location of interest. The probe comprises a lens fitted onto a coherent bundle of glass fibres which brings the fluorescent image out of the body cavity. Alternatively, the probe can be a rigid endoscope without fibre optics. The image brought out of the body is captured by very sensitive photodetectors and further processed in an attempt to delineate diseased tissue on the basis that diseased tissue has a different fluorescence intensity than normal tissue. This process loses significant amounts of light due to the image transfer through the optical fibres and a great deal of effort in developing new fluorescence endoscope systems is directed toward new ways of acquiring, enhancing and analyzing the very faint fluorescence images to extract as much information as possible.
An example of an endoscope system that relies on this foregoing general arrangement is disclosed in applicant's co-pending patent application Ser. No. 07/725,283 filed Jul. 3, 1991, now abandoned in favour of continuation application Ser. No. 08/082,019 now abandoned in favour of Ser. No. 08/428,494 which is still pending. Other work has been done in this field that attempts to exploit the different fluorescence spectra of diseased and healthy tissue. U.S. Pat. Nos. 5,131,398 to Alfano, U.S. Pat. No. 4,930,516 to Alfano, and U.S. Pat. No. 4,786,813 to Svanberg et al. disclose various equipment and methods that acquire and process fluorescence images in an attempt to detect and delineate diseased tissue.
At present, many fluorescence endoscopy systems rely on drugs with strong fluorescence signals as well as very sensitive detectors to detect the emitted fluorescence signals. In applicant's co-pending patent application Ser. No. 08/082,019 filed Jun. 23, 1993, it was shown how tissue autofluorescence alone, without drugs, can be used to detect diseased tissues such as early cancer and others. In this approach, very faint fluorescence signals must be measured at two or more specific wavelengths of emitted fluorescence. To accomplish such a measurement, very sensitive detectors, such as image intensified cameras operating at the limit of their working range must be used. This arrangement works well for detecting some tumours, but care must be taken to correctly adjust the autofluorescence signals to create appropriate images with minimal noise in order to recognize the diseased site under examination. Also, image intensified cameras (or similar detectors) are very large, they employ high voltage circuitry and they cannot be made to fit the end of the endoscope. The fluorescence images must therefore be brought out of tissue cavities through the coherent optical fibers of the endoscope before processing of the images and/or displaying them on a video monitor.
It has been suggested that the acquired fluorescence image of endoscope systems would be of better quality if it could be collected by a sensor at the end of the endoscope probe inserted into the body. The outer diameter of an endoscope probe must be small to allow insertion into various body cavities thereby limiting the size of the sensor that can be mounted at the distal end of the apparatus. As previously stated, fluorescence images are generally extremely faint and it is not possible for these small image sensors to capture the fluorescence images. Theoretically, increasing the excitation irradiance would increase the fluorescence intensity, however, this may also result in unwanted thermal damage or photobleaching of the tissue under examination.
Prior art endoscopes have been developed that permit the image sensor to be located at the tip of the endoscope probe, however, in general, this endoscope equipment is intended for collecting reflected light and is not suitable for reliably capturing faint fluorescence images.